1. Field of the Invention
The present invention relates to an electromagnetic coupler suitable for use in wireless communications systems to communicate information between wireless terminals disposed in close proximity to each other by using a longitudinal electrostatic field or a longitudinal induced electric field. The invention also relates to a wireless terminal including said electromagnetic coupler, and a method for designing electromagnetic couplers.
2. Description of Related Art
Conventional electromagnetic couplers include one disclosed in JP-B 4345851. This electromagnetic coupler (high-frequency coupler) can communicate over a short distance by using a longitudinal electrostatic field or a longitudinal induced electric field. Also, a large volume of data can be exchanged between wireless terminals equipped with this electromagnetic coupler in UWB (Ultra Wide Band) communications systems, in which wide band signals are used.
Also, in the above wireless communications system, each of the electromagnetic couplers included in the wireless terminal on a transmission side (transmitter) and the wireless terminal on a reception side (receiver) is formed by connecting a plate-like electrode to a series inductor and a parallel inductor via a feeder (high-frequency transmission line). When the electromagnetic couplers are disposed such that they face each other, the constant of a capacitor formed by the two electrodes, the series inductors, and the parallel inductors are set up such that impedance matching can be obtained in the frequency band of interest in wireless communications. As a result, the electromagnetic couplers facing each other behave like a band pass filter as a whole in the frequency band of interest, making it possible to efficiently transmit high-frequency signals between the two electromagnetic couplers.
As described above, a conventional electromagnetic coupler (e.g., described in JP-B 4345851) includes a plate-like electrode and series and parallel inductors. The design parameters of a conventional electromagnetic coupler are the size of its electrode and the values of its series and parallel inductors. Chip parts can be used as the series and parallel inductors. Unfortunately, however, common chip parts can be lossy and relatively costly depending on the frequency band of interest, while low-loss chip parts are even costlier.
A conventional electromagnetic coupler can also be formed of a general-purpose printed-circuit board instead of chip parts. In this case, its series and parallel inductors are formed of transmission lines such as microstrips. Forming an electromagnetic coupler with a printed-circuit board can control loss increase and reduce the cost compared to the case with chip parts. In this case, however, the design parameters are the width, length, and configuration of the transmission lines forming the series and parallel inductors in addition to the size of its electrode.
In the case of conventional electromagnetic couplers facing each other, a capacitor with an appropriate capacitance value is obtained between the electrodes disposed at a certain distance from each other, and communication is made possible at a certain frequency. When a capacitor with an appropriate capacitance value is not obtained, communication is made difficult. However, when wireless terminals each including an electromagnetic coupler are disposed such that they face each other, a capacitor with an appropriate capacitance value may not be obtained depending on the distance between the electrodes of the electromagnetic couplers. For example, even when wireless terminals are disposed such that the distance between the electrodes of their electromagnetic couplers becomes the shortest, it is difficult to obtain a desired capacitance value if the distance between the electrodes of the electromagnetic couplers is relatively long due to the structure of the wireless terminals or other factors. In such a case, since a capacitor with an appropriate capacitance value is not obtained, communication is made difficult. Although it is possible to adjust a capacitance value by changing the area of an electrode, it is difficult to change the area of an electrode due to constraints on the structure of an electrode, such as a constraint that the electrodes facing each other must have the same area.
Moreover, in communication by using electromagnetic couplers, communication directions need to be increased in some cases. Communication directions can be increased by increasing the area of an electrode to produce the electric field in a wider range. In conventional electromagnetic couplers, however, it is difficult to increase communication directions, because an appropriate capacitance value between electrodes needs to be obtained and there are constraints on the structure of an electrode.